1. Field of the Invention
This invention relates to manganese dioxide which is used for anode active material of a lithium primary battery, and a method of producing the same.
2. Description of Related Art
Manganese dioxide, carbon fluoride, etc. have been known as representative anode active material of a lithium primary battery, and these materials have been already practically used. Of these anode active materials, manganese dioxide has particularly excellent discharge characteristics and is low in price.
In order to mainly improve preservation of the lithium primary battery, the manganese dioxide used for the anode active material of the lithium primary battery has been hitherto produced by a method as disclosed in JPB Sho 57-4064. That is, the manganese dioxide is calcined at 350.degree. C. to 430.degree. C. to remove water in the manganese dioxide, and then it is used as anode active material. At present, the lithium primary battery using manganese dioxide as anode active material has been utilized in a camera or the like, and recent improvement in performance and function (higher performance and higher functionality) of the camera increasingly requires a lithium primary battery having more improved discharge characteristic and preservation characteristic.
In view of such a condition, it has been recently recognized that manganese dioxide after calcined (hereinafter referred to as "calcined manganese dioxide") is required to have a large specific surface area to improve not only the discharge capacity of the lithium primary battery, but also the discharge voltage thereof.
Furthermore, it has been hitherto considered that in order to keep the specific surface area of calcined manganese dioxide high, it is necessary to perform a calcination treatment in a low calcination temperature range of 350.degree. to 430.degree. C. as described in the above prior art, particularly in a range of 350.degree. to 400.degree. C. However, the low temperature calcination treatment increases the amount of residual bound water in calcined manganese dioxide although the specific surface area of the manganese dioxide is kept high. When such manganese dioxide having a large amount of residual bound water is used as anode active material for a lithium primary battery, the residual water reacts with lithium or lithium alloy at a cathode and decomposition of dehydrated electrolytic solution is promoted. Therefore, the preservation performance of the lithium primary battery, particularly the performance of the lithium primary battery after preserved at a high temperature is remarkably deteriorated.
On the other hand, if the calcination temperature is increased to 440.degree. C. or more to improve the preservation characteristic, the amount of the bound water is reduced, however, manganese dioxide is decomposed and changed to Mn.sub.2 O.sub.3, so that the discharge performance of the battery is remarkably deteriorated.
As described above, there has not yet been developed a method of manufacturing calcined manganese dioxide for a lithium primary battery in which the high specific surface area and the low bound water amount can be kept at high level with excellent balance.
An object of the present invention is to provide manganese dioxide for a lithium primary battery and a method of producing the same in which both suppression in reduction of specific surface area and removal of bound water can be performed at the same time.